The Study to Help the AIDS Research Effort (SHARE) along with 3 other clinical sites (Northwestern University, Chicago IL, UCLA School of Public Health, Los Aneles, CA, Pittsburgh-Graduate School of Public Health, Pttsburgh, PA) collectively known as MACS was funded in 1983 and began recruiting AIDS-free homosexual and bisexual men in 1984 for a natural history study of AIDS, subsequently redefined as the natural history of HIV-1 infection. The origianl goals of the MACS were to define risk factors for acquisition of HIV-1 infection, mechanisms of AIDS pathogenesis, and specific incidences and risks factors for particular clinical manifestations of HIV-1 infection, such as infections, malignancies and neuropsychological impairments. The SHARE cohort consists of 1153 men who were recruited in 1984, 240 "new recruits" who were enrolled in 1987-1990, and 54 people who have transferred from other MACS centers, for a total of 1447 men. Overall, the retention in SHARE of active participants who have not died or been censored (seronegatives only) from the study has been 73% for seronegatives and 96% for seropositives. The majority of these participants are seen in clinic which takes place two evenings a week. At the clinic visit a number of interviewer- administered and self-administered instruments are completed. Participants are examined by a clinician and have blood specimens drawn. Currently there are 283,580 aliquots of these specimens in the local freezers, about 200,000 of which are serum, plasma or PBMC.